1. Field of the Invention
The present invention relates to a polyaniline-containing solution and method for preparing the same, more particularly to an optically transparent polyaniline-containing solution that is formed in-situ upon polymerizing on aniline monomer, and a method for preparing the same.
2. Description of the Related Art
Conductive paints have been widely used for anti-static electrostatic discharge, electromagnetic interference (EMI) shielding, and insulated plastic materials.
One type of the conductive paints is prepared by blending a powdery metal, such as Zn, Ni or Ag, with a resin. However, this type of conductive paint suffers from several disadvantages. First, due to the high density of metal powder present in the conductive paint, the metal powder will tend to sink to the bottom of the conductive paint whenever the agitation of the paint is insufficient or fully stopped. Agitation of the conductive paint is required in order to keep a uniform concentration for the paint during painting. Second, when the coated layer of the conductive paint is dried in an open environment, particles of the metal powder present in the conductive paint tend to be gradually precipitated out and coalesce together, thereby resulting in a gradual decrease in quality. Third, the powdery metals are prone to be oxidized when the paints are exposed to air, and consequently become electro-insulated in the long run. Finally, the cost of using powdery metals in the conductive paints are expensive, and the powdery metals may become harmful to the environment as residual wastes in the plastics which they were blended or coated with.
Polyaniline has emerged as one of the promising conducting polymers and can be used in a variety of applications, such as paint, antistatic protection, electromagnetic protection, electro-optic devices such as liquid crystal devices (LCDs) and photocells, transducers, circuit boards, etc. However, processing of polyaniline into useful products or devices as described above has been problematic because of its insolubility in common solvents.
Typically, polyaniline is produced as solid emeraldine salt from chemical oxidative polymerization in the presence of protonic acids such as HCl and H.sub.2 SO.sub.4. The solid emeraldine salt is normally precipitated out in the reaction mixture during the polymerization. Hence, this solid salt has to be recovered from the reaction mixture, and subjected to filtering, washing with water and solvent, and drying in order to yield the solid polyaniline for use in the applications described above. The polyaniline obtained in such way is normally insoluble, which hinders the application of the polyaniline.
PCT US92 Patent No. 04167 disclosed the use of functionalized protonic acid in the preparation of conductive polyanilines to induce processability and solubility for the conductive polyanilines in an organic solvent.
Harlev et al., U.S. Pat. No. 5,618,469, disclosed a method for preparing a processable solution of emeraldine salt for use in the preparation of electrically-conductive and optically transparent coatings, and to articles, such as liquid crystal devices and other electro-optic devices. Such method comprises oxidatively polymerizing aniline monomer in the presence of a protonic acid, for example pyruvic acid, to yield a solid emeraldine salt, reacting the solid emeraldine salt with an aqueous ammonium solution to form a solid emeraldine base, and dissolving the emeraldine base in an additional amount of pyruvic acid to form a solution of polyaniline.
Some disadvantages of the method described above are that the emeraldine salt formed in the reaction mixture during the oxidative polymerization is insoluble and precipitated out so that a complex process is needed to transform the salt into a solid emeraldine base which is then dissolved in pyruvic acid to form into a processable solution of polyaniline.
Smith et al., U.S. Pat. No. 5,470,505, disclosed that the emeraldine salt prepared by standard methods of oxidative polymerization of aniline monomer in the presence of a protonic acid can be dissolved in an acid, particularly strong acid such as concentrated H.sub.2 SO.sub.4, CH.sub.3 SO.sub.3 H, CISO.sub.3 H, CF.sub.3 SO.sub.3 H and HNO.sub.3 (70% or fuming). The emeraldine salt (polyaniline) dissolved in one of these acid solutions is then processed into desired articles in the applications.
Abe et al., U.S. Pat. No. 5,728,321, disclosed a solution of polyaniline (dissolved in an aprotic polar solvent, such as N-methyl-2-pyrolidone) in doped state can be obtained by a method using a specific protonic acid, such as hydrofluoroboric acid, hydrofluorophosporic acid, perchloric acid, or any other organic acids having acid dissociation constant pKa values of less than 4.8, as dopants in the oxidative polymerization of aniline monomer. Also, the polyaniline obtained according to the above method, which is insoluble in an organic solvent, can be dissolved in an aprotic polar solvent in an undoped state. The undoping of doped polyaniline in order to permit the polyaniline to be soluble in organic solvent is burdensome and increases production cost.
Traditional methods of preparation of polyaniline in a processable form, including the prior arts mentioned above, have to go through the processes of recovering, filtering, washing, and drying of the reaction product to obtain the solid polyaniline due to the insolubility of the polyaniline formed in the reaction mixture, and need additional processes, such as transforming the emeraldine salt into emeraldine base and dissolving the solid polyaniline or emeraldine base in a solvent, to obtained the desired solution of polyaniline.